Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - A Forcool question

For some time now I've been experiencing the following cooling system problem with my 1500 midget.

Under normal running conditions everything is fine,.but once the car is stationary and the engine switched off, coolant is forced out of the expansion tank filler cap.

To find out what is happening I have started the engine from cold with the expansion tank pressure cap removed. The expansion tank was filled to the correct level and as the engine warmed up,the coolant level dropped, suggesting that it was being drawn back into the engine. Once the engine was fully hot it was switched off whereupon the coolant level rose in the expansion tank to the point where quite a lot overflowed onto the floor.

As far as I can see, this is the opposite of what is supposed to happen, i.e the coolant expands into the tank when the engine is hot and is drawn back into the engine as it cools.

Any suggestions would be welcome.



Heat soak? The circulation of the water through the rad when it is running keeps temp down (even if stationary). When you switch off the stored heat in the block soaks back into the coolant increasing its temperature and volume?

Or the cap is knackered?

My tuppence-worth...
L McInally

" suggesting that it was being drawn back into the engine"

Where to? IMHO there shouldn't be any voids for it go - unless you have an airlock?

J Smith

Now if you were using waterless snake oil this wouldn't happen hahahahahahah
Seriously as per Jim you may have an airlock. Bleed it from the plastic cap on the thermostat housing by squeezing the top and bottom hoses (my method, may not agree with the "Book/s" but it seems to work)
R W Bowers

When the engine is switched off the coolant is no longer being pumped around the engine. The engine is still hot so the coolant absorbs the heat, it expands and goes out of the expansion tube into the expansion tank.

if you watch the temp gauge when you switch the engine off you can see the temp rise quite quickly for a few minutes after switching off.

i have my elec fan on a thermo switch direct from the battery (via a big red isolator key) this will run on and off for a good 5 minutes after engine is switched off if i let it , there is some coolant cirulation due to thermosyphon and the fan running actually encourages this...

i would say, yes you may have an airlock, but what you are experiancing is the cooling system working as it is supposed to...

ETA, sorry, just realised ive repeated what LMcinally said...
Andy Phillips (frankenfrog)

as per L McInally, heat soak and is the cap OK? - if you're not sure for about 3 I'd buy a new one (15 lb)

the Driver's Handbook details of how to get air out of the system on refills, if you want to check for airlocks you could follow that -
. on cold engine and coolant
. take the hose off the expansion tank and have it held higher than the radiator
. top up coolant through the (engine) filler plug hole to bottom of threads
. reconnect hose to expansion tank
. screw filler plug back in
. top up coolant in expansion tank to half way and put pressure cap back on
. run engine up to temperature
. turn engine off and allow coolant system to fully cool
. top up coolant in expansion tank to half way

that should get your coolant back to level and take out airlocks if they were there
Nigel Atkins

Thanks for the comments folks.

Up until recently the cooling system had been filled with Nigel's favourite, Forlife (snake oil?) but it was proving to be an expensive hobby due to the amount of coolant that was being lost. Now we are back to a 50/50 water and a/f mixture and it remains to be seen how that performs.

In the meantime a new pressure cap may be the order of the day and another top-up to get rid of any airlocks.


B - Airlocks seems to be it. They seam to appear in my system from time to time. Usually a heater circuit problem. Had one today. Stopped heat from heater, engine hot on gauge. Arrived at destination. Allowed engine to cool (not cold). Undid thermostat nut till coolant wept a bit. (not flowing) Retighten. Then toastie tootsies (or burnt ones) once more.

The air lock must have been very small and just in the wrong place.

Good luck.
Dave Squire 1500

I tend to not use the 4-LIFE until I think the cooling system is sound - it's after that that things ususally happen like, on various cars, water pump lets go, hose splits, head gaskets, wrong cooling fan is sent and fitted, engine has to come out again because of p*ss poor work by 'specialist experts'

I've bought so much over the years that I've got a part reel of the labels, because after I've put it in I've needed more for the above reasons my mate once said I should call it 4-NOW

but I put the 4-LIFE in as soon as possible and stick with it because it's helped me find leaks and helped minimise the damage when I've not realised HGF

I've never had airlocks simply by following the directions in the Driver's Handbook :p

and I've had plenty of practice with my present Midget
Nigel Atkins

being serious you should get all the air out, the way you're patching things at the moment you could be just getting rid of a little and possibly even adding a little

get all the air out by static running up engine to temperature and beyond the hose off the expansion tank and held higher than the radiator, then top up as required following the procedure - or do what ever running with pressure cap off/squeezing hoses/whatever procedure you prefer to get all the air out
Nigel Atkins

This thread was discussed between 26/02/2014 and 28/02/2014

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.